The research described in this proposal involves studies of the structure of ligands near metal ions, electron spin relaxation times, and metal-metal separations in a series of biologically important metal complexes. Ligand structures will be determined using low temperature angle selected endor spectroscopy. This is a new technique which has been developed in our laboratory which allows one to obtain single crystal like data from randomly oriented solids. Angles are selected by irradiation of the esr transitions of molecules held at fixed angles with respect to an external magnetic field. The nmr spectra of these molecules are taken and nmr shifts are analyzed to yield dipolar and Fermi contact interactions. The dipolar interactions are used to determine the position of nuclei with respect to the metal ion. The overall structure of ligands near the metal ion can be determined with this technique. Structural studies of this type will be conducted with a series of metal containing proteins, hemoglobins, and nucleic acids. A new technique has been developed to measure electron relaxation times of transition metal complexes. These relaxation times depend on the electronic structure of the metal complex and exchange and dipolar interactions between the spins of different metal atoms. Relaxation measurements will be conducted with a series of complexes to obtain information on electronic structures and metal-metal separations. The combined study of ligand structures, electronic properties and metal-metal separations will provide valuable new information on the structure of these complexes. Structural information of this type is necessary for an understanding of the mechanism by which these metal complexes function in a variety of biological processes.